1. Field of the Invention
The invention is related generally to the field of interpretation of measurements made by well logging resistivity instruments for the purpose of determining the properties of earth formations. More specifically, the invention is related to an apparatus and method for determination of the electrical resistivity of an earth formation when there is an azimuthal variation in the resistivity.
2. Background of the Art
A well logging device with electrodes that has been commercially available for many years and that is known under the name “Dual Laterolog” is described in U.S. Pat. No. 3,772,589 to Scholberg. That device comprises an array of annular electrodes used for sending electrical measurement currents into the formations for the purpose of measuring their resistivity. The measurement currents are focused in an annular zone having the form of a disk perpendicular to the borehole axis by means of auxiliary currents emitted by guard electrodes. That device includes measuring deep resistivity of the earth formations and measuring shallower resistivity of the earth formations by emitting currents at different frequencies, typically 35 Hz and 280 Hz.
A drawback of the Scholberg device is that its longitudinal resolution is poor, being about one meter. In addition, it does not have azimuthal sensitivity. Azimuthal variations of resistivity can occur in boreholes that are deviated or horizontal. When this happens, the annular zone scanned by the device around the borehole comprises different layers of earth formation and gives an average measurement that has little meaning. In addition, it is well known that by the time wireline logging devices are run in a borehole, there may be invasion of the earth formations by drilling mud from the borehole. This results in an invaded zone in which the resistivity is different from that of the uninvaded earth formations. When there is an azimuthal variation in the depth of the invaded zone, there will be azimuthal variations in resistivity. It is therefore desirable to obtain resistivity measurements in a plurality of azimuthal directions around the borehole.
British patent GB 928 583 to Threadgold et al. discloses an array of azimuthal measurement electrodes distributed circumferentially around the periphery of a logging sonde. A guard electrode which surrounds the measurement electrodes enables an auxiliary current to be emitted for focusing the currents emitted by each of the measurement electrodes. In such a sonde, measurement current focusing is passive, and this focusing is obtained by emitting the various currents via electrodes that are short-circuited together. This suffers from the drawback that the focusing is not particularly effective. In addition, the Threadgold device also has poor longitudinal resolution.
French patent FR 2 611 920 to Mosnier describes a logging sonde in which correction means are proposed acting on the potential of the current electrodes in order to improve focusing. The sonde includes monitor electrodes disposed at a certain distance ahead of the current electrodes and circuits that are responsive to the potentials detected by said monitor electrodes to control the measurement currents. It is difficult and complicated to make such a sonde, which requires concentric rings of electrodes.
U.S. Pat. No. 5,399,971 to Seeman et al. attempts to address the drawbacks with the Threadgold and Mosnier devices by a modification of the basic laterolog configuration. A modification of the Seeman device is disclosed in Smits et al., (SPE paper 30584) and illustrated schematically in FIGS. 4a and 4b. The overall electrode arrangement resembles that of the Dual Laterolog. The electrodes AO, A1 and A2 with their symmetric counterparts AO′, A1′ and A2′ serve to emit current into the formation, while monitoring electrodes M1, M2 and A1* along with M1′, M2′ and A1*′ are used to measure potentials. In addition, the central section of the array incorporates twelve azimuthal electrodes to add the directional capability. The AO electrode is split into two sections. AO and AO′ are located on either side of the azimuthal array with the two monitoring electrodes A0* and A0*′ added at their respective centers.
The devices disclosed in Seeman and in Smits are still susceptible to errors due to poor azimuthal focusing. One reason is that there is an offset between the azimuthal array of electrodes and the borehole wall. In the presence of a conducting mud (water based mud), focusing in the azimuthal direction becomes problematic. This results in a reduced azimuthal resolution of the resistivity measurements.
In addition to azimuthal resistivity measurements, there are many prior art devices that make so called “microresistivity” measurements. Such a device using arrays of electrodes on pads is disclosed in U.S. Pat. No. 6,348,796 to Evans et al., having the same assignee as the present invention and the contents of which are fully incorporated herein by reference. Disclosed in Evans '796 are button electrodes on a pad that makes contact with the formation or is in close proximity to the formation. Microresistivity measurements when made with an array of electrodes can provide high resolution resistivity images of the borehole wall.
Besides the laterolog type devices described above, a pad mounted azimuthal resistivity device has been discussed in U.S. Pat. No. 6,025,722 to Evans et al. The Evans '722 device uses electrodes mounted on pads. Bucking or focusing currents are provided from the body of the tool. Additional focusing may also be provided by use of additional circuitry for providing focusing from the body of the tool. The Evans '722 device does not provide a capability of providing different depths of investigation.
It would be desirable to have a method and apparatus for making azimuthal resistivity measurements of earth formations with a logging tool that addressed the problems discussed above. Such a method and apparatus should preferable be able to provide high resolution resistivity measurements. The present invention satisfies this need.